Fluorescent lamp with protective coating

ABSTRACT

In a fluorescent lamp which is coated internally with a phosphor, a protective coating of fine alumina particles of submicron size is applied over the phosphor layer as a porous discontinuous coat. The protective postcoating improves maintenance and reduces end discoloration of the oxide ring type. It is most beneficial in lamps having relatively poor maintenance such as green zinc silicate aperture lamps and very highly loaded calcium halophosphate lamps.

United States Patent Hammer et al.

FLUORESCENT LAMP WITH PROTECTIVE COATING Inventors: Edward E. Hammer,Mayfield Village; Edward E. Kaduk, Lyndhurst, both of Ohio GeneralElectric Company, Schenectady, NY.

Filed: Sept. 17, 1973 Appl. No.: 397,913

Related U.S. Application Data Continuation-impart of Ser. No. 188,096,Oct. 10, 1971, abandoned.

Assignee:

U.S. Cl. 313/486; 313/113; 313/488; 313/489 Int. Cl. H01] 61/42 Field01' Search 313/109, 113, 220, 221, 313/489, 488, 486; 117/335 LReferences Cited UNITED STATES PATENTS 6/1940 Breadner et a1. 313/109 11 May 27, 1975 2,213,796 9/1940 Zecher et 313/109 2,238,784 4/1941 Scottct a1 313/109 2,331,306 10/1943 Casellini 117/335 X 2,386,277 10/1945Smith 313/109 3,067,356 12/1962 Ray 313/109 X 3,275,872 9/1966 Chemin etal..... 313/109 3,599,029 8/1971 Martyny 313/109 3,636,352 l/1972Wanmaker et a1. 313/109 X Primary Examiner-Palmer C. Demeo Attorney,Agent, or Firm-Ernest W. Legree; Lawrence R. Kempton; Frank L. Neuhauser[57] ABSTRACT In a fluorescent lamp which is coated internally with aphosphor, a protective coating of line alumina particles of sub-micronsize is applied over the phosphor layer as a porous discontinuous coat.The protective postcoating improves maintenance and reduces enddiscoloration of the oxide ring type. It is most beneficial in lampshaving relatively poor maintenance such as green zinc silicate aperturelamps and very highly loaded calcium halophosphate lamps.

5 Claims, 2 Drawing Figures PATENTEBMY 27 1915 r Qk n Smu T m O Q KJJ ini n A E d F vw a Mm T E FLUORESCENT LAMP WITH PROTECTIVE COATING Thisapplication is a continuation-in part of our earlier copendingapplication Ser. No. 188,096 filed Oct. l0, l97l. similarly titled andassigned and now abandoned.

BACKGROUND OF THE INVENTION The invention relates to fluorescent lampswherein a low pressure discharge through mercury vapor producesultraviolet radiation which excites a phosphor coated internally on theenvelope walls to produce light.

It is well-known that the light output of the usual fluorescent lampdecreases during the course of its life. Various factors contribute tothe drop-off in light output during operation. Some contributing causesare deposits of impurities from the cathode and formation of oxides ofmercury, changes in the phosphor itself, and changes in the glassparticularly where it is subject to ultraviolet radiation causing adecrease in transmission. The ability of a fluorescent lamp to resistdrop-off in light output during life is generally termed maintenance,and it is measured as the ratio of light output at a given life spancompared to initial light output and expressed as a percentage.

The more common fluorescent lamps have excellent maintenance. Forinstance, white 40-watt fluorescent lamps for ordinary lighting havemaintenance as high as 85 percent at 20,000 hours oflife. However, otherfluorescent lamps are not nearly as good. A green zinc silicate aperturelamp used for xerographic reprography may have lumen maintenance nobetter than 65 percent at 100 hours. In these lamps the fluorescentcoating is applied over a reflector coating and the coatings extend onlypart way around the circumference of the envelope leaving alongitudinally extending clear strip or aperture through which the lightis emitted. Other fluorescent lamps having poorer maintenance areextremely highly loaded lamps of both circular and noncircular crosssection in which the power input ranges up to 50 watts per foot length.

SUMMARY OF THE INVENTION The objects of the invention are to improve thelumen maintenance in fluorescent lamps where the maintenance tends to below, and to reduce end discoloration.

In accordance with our invention, we have found that lumen maintenancein such lamps may be improved by postcoating with aluminum oxide whereina thin layer of fine alumina is applied over the phosphor layer,suitably as a suspension in a binder. Both the phosphor layer and the N0,, postcoat may be lehred in a single operation in the same way as thephosphor layer alone would be lchred.

The aluminum oxide postcoat in accordance with our invention isbeneficial in two ways. In lamps wherein lumen maintenance tends to bevery low, as in green zinc silicate aperture lamps for reprographicapplications, the postcoat achieves a remarkable improvement inmaintenance, as much as sixfold. In other lamps wherein lumenmaintenance is not so low but which are subject to end discoloration,the postcoat achieves an improvement in end discoloration and asubstantial reduction in oxide ring darkening.

DESCRIPTION OF DRAWING FIG. 1 shows an aperture fluorescent lamp havingan A1 0 postcoat according to the invention.

FIG. 2 is a cross section through the lamp to an enlarged scale showingthe various layers internally deposited on the glass.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. I, there is showna fluorescent lamp I comprising an elongated soda lime silica glass tube2 of circular cross section. It has the usual electrode 3 at each endsupported on inlead wires 4.5 which extend through a glass press 6 in amount stem 7 to the contacts of a base 8 affixed to the end of the lamp.The sealed tube is filled with an inert gas such as argon or a mixtureof argon and neon at a lower pres sure, for example 2 torr, and a smallquantity of mercury, at least enough to provide a low vapor pressure ofabout six microns during operation.

As best seen in FIG. 2 showing a section through the tube wall, theinner surface of the glass tube is protectively coated with a thin clearfilm 9 consisting of titanium dioxide. To form this clear layer, ametalloorganic compound of titanium such as tetrabutyl titanate ortetraisopropyl titanate dissolved in an appropriate solvent such asbutyl alcohol or butyl acetate, is applied to the glass. The solventevaporates almost upon application and the titanate is left depositedupon the inner surface of the glass tube. Moisture from the airhydrolizes the titanate almost as fast as the solvent evaporates formingtitanium dioxide which remains as a very thin clear continuous film in athickness from 0.002 to 0.02 microns.

The illustrated lamp is provided with a reflector coating 10 andthereover a coextensive phosphor coating 11, both coatings extendingaround the major portion of the glass tubes circumferential surface.This leaves a narrow uncoated strip or aperture 12 extending lengthwiseof the lamp. The coatings 10 and 11 may be applied at first over theentire glass tube internal surface and then scraped or brushed off toform the aperture 12 in the desired width, for instance over a 45portion of the circumference of the tube. A preferred material for thereflective layer is a particulate coating of titanium dioxide having aparticle size less than 1 micron, for instance centering on about 0.3micron which is about half the median wavelength of the visiblespectrum. The TiO may be applied as a suspension in a solution of ethylcellulose in an organic solvent to serve as a binder, the suspensionbeing drawn up into the tube supported vertically and then allowed todrain out. Thereafter the tube is lehred in order to decompose and driveout the organic binder. Alternatively the reflector coat may consist ofmagnesium oxide MgO.

The phosphor consisting of zinc orthosilicate Zn SiO. is next applied asa suspension in a solution of nitrocellulose in butyl acetate which isdrawn up into the tube and allowed to drain out. At this stage the cleararea or aperture 12 is scraped out in the desired width. The scrapingremoves the relatively thick powdery reflective layer of TiO andphosphor layer of Zn SiO but the clear protective layer of TiO whichresulted from the hydrolysis of tetrabutyl titanate is very adherent andis not affected. Instead of applying the coating over the entireperiphery and then scraping, an alternative method is to introduce apool of suspension of the desired coating in a horizontally supportedtube which is then rocked back and forth to achieve the desired angularwidth of reflective coating. followed by drying and lehring, as taughtin US. Pat. No. 2,892,440 Fulton et al.

In the case of a very highly loaded fluorescent lamp for generalillumination the reflective layer is omitted and the phosphor layer isapplied directly over the clear protective film of titanium dioxide. Aphosphor commonly used for white fluorescent lamps is calcium halophosphate activated with manganese and antimony and it is commonlyapplied as a suspension in a water soluble binder.

In accordance with our invention, a postcoat 13 consisting of AI Oparticles in a size less than l micron is applied over the phosphorcoat. The A1 particles may be applied as a suspension in a solution ofethyl cellulose in an organic solvent serving as a binder. Thesuspension is brought up into the tube while vertically supported andthen allowed to drain out and dry. The tube is then lehred at atemperature from about 550C to 600C for 3 to 5 minutes to decompose anddrive out the binder of both the phosphor layer and the alumina postcoatlayer thereover. The lehring procedure is the same as is conventionallyused in regular fluorescent lamp production without postcoat, so that noadditional burden is imposed thereby on the manufacture of the lamp.

By way of example of postcoating technique, a relatively thicksuspension may be prepared by dispersing 200 grams of A1 0 powder of0.02 micron average particle diameter pre-fired at 1100C, in 5.6 litersof binder comprising 100 grams of ethyl cellulose dissolved in thinnerconsisting of equal parts by volume butyl acetate and naphtha. A ballmill or a suitable high speed dispersion mill (Kady mill) may be used.Before application, the foregoing thick suspension is thinned down bydiluting 100 cc thereof with binder comprising 25 grams of ethylcellulose in 740 cc of the same thinner. This thinned suspensioncontains about 4.24 mg A1 0 per cc and about 3.25 cc are retained in the18 inch T8 aperture fluorescent lamp previously described, correspondingto 14 Mg AI O per bulb or 40 micrograms per cm of bulb surface. Weprefer to have the postcoat extend over the clear aperture, as shown inFIG. 2 of the drawing.

Table I below compares the light output of 18 inch T8 green zincsilicate aperture lamps, some postcoated with M 0 as previouslydescribed, and others similar in all respects except not postcoated.serving as control.

The marked improvement in maintenance of the postcoated lamps isapparent. Lumen maintenance calculated as the ratio of light output at300 hours relative to light output at V2 hour. is 7i percent for thepostcoated lamp. as against 51 percent for a control lamp similar in allrespects except for the absence of the postcoat. The comparison has beenmade on the basis of the /2 hour rather than the zero hour figure toavoid the very rapid drop-off during the first minutes of operationwhich distorts the maintenance figures and has no practicalsignificance.

Tests on very highly loaded fluorescent lamps wherein the power inputranges up to 50 watts per foot length show similar improvements inmaintenance as a result of postcoating. In one series of lamps whereinmaintenance was percent at 3000 hours life, postcoating raised themaintenance to percent.

The improvement in end discoloration clue to oxide rings made possibleby the use of a postcoat according to the invention is apparent in thefollowing Table 2 comparing end discoloration in cool white fluorescentlamps operated with frequent starts at a loading of 10 watts per foot.The comparison is made in demerit points wherein 1 represents barelynoticeable graying, and 10 represents heavy blackening all around.

It is apparent from the table that barely noticeable graying, whoseonset occurs before 1000 hours in the control lamps, is delayed to 3000hours by postcoating.

The thickness of coating applied over the phosphor is very difficult tomeasure because both phosphor particles penetrate into the voidsthroughout the phosphor. The coating thickness for a particularsuspension of A1 0 depends on phosphor particle size and coating textureor laydown characteristics, and the weight of A1 0 per unit area is moreeasily measured. The effect of weight of postcoat on window brightnessand lumen maintenance in 18 inch T8 green zinc silicate aperture lampsis given in Table 3 below.

TABLE 3 Coating Weight Light Output Maintenance Mg/Bulb G/cm 0.5 hr. 100hr. 300 hr. 500 hr. 500/1/2 None-control 0 69l 517 422 380 55.0% 30 667569 503 48] 72.1% 20 60 652 505 460 47 I 72.2% 30 601 3l8 225 TABLE I Asmay be expected, an excessive weight of A1 0 is not good, due in part tothe greater difficulty of prep Mum erly lehring the underlying layers.The optimum weight T85! Light Output [finance 65 of N 0,, postcoatingwill vary with the phosphor and 0 hr l/u hr. m. 300 hr. 300/1 ,2 lampcombination. We have found the desirable range Post to extend from l0 tomicrograms per cm of bulb coat 130.5 127.1 98.4 so 71 7, surface. andprefer a roximately 40 micrograms/cm Con for the l8l h T [ml [3346 in?8M 65 51% no 8 green zinc silicate aperture lamp previously described.

The colloidal aluminum oxide postcoat in accordance with the inventionis discontinuous, that is porous and pervious, rather than continuous,nonporous and impervious. This result follows necessarily from theweight or quantity of M applied as previously de scribed herein, and itsmethod of application by deposition out of a liquid suspension. It iswell-known that a phosphor coating is not a smooth uniform dense coatingbut consists of protuberances and cavities, or hills and valleys.According to published data, phosphor particles as used in lamps vary insize from a few to several microns, and 4 microns may be taken astypical for a zinc silicate phosphor. In order to have a phosphor filmwithout bare spots, the phosphor layer must be several particles thickand the top layer of particles will project at least 4 microns above themass. If a projection of 5 microns is assumed, this determines thethickness or depth of M 0 coating that must be applied in order to havea continuous coat or impervious barrier.

Suppliers of colloidal A1 0 powder having 0.02 micron average particlediameter (v.g. Cabot Corp.) give the density of the material as 3.6grams per cc. Thus for instance where 40 micrograms of colloidal aluminaare applied per square centimeter, the volume of this quantity will be40 X gm/3.6 gm/cm l.] X 10 cm Assuming that the upper 5 micron layer ofphosphor is half filled with phosphor particles, then the volume of Alonneeded for complete filling of the empty spaces to assure completecoverage of the phosphor particles will be 5 X l0/2 cm 2.5 X 10 cm.Comparing the volume of Alon supplied to the volume required forcomplete coverage, the ratio is seen to be 1.1 X NY /2.5 X l() 4.4 X 10In other words when 40 micrograms of colloidal alumina are provided persquare centimeter of phosphor coated surface, there is 4.4 percent ofthe quantity needed to provide complete coverage of the phosphorparticles.

In fact, there is not even that much coverage because the hill andvalley effect of the phosphor particles is appreciably greater than the5 micron thickness that has been assumed. For instance, W. Elenbaas inLight Sources says that the phosphor film has to be 3 mean particlediameters thick in order to have no bare spots on the glass not coveredby phosphor. Thus, a hill and valley depth of 12 microns is needed toavoid bare spots and on that basis the percentage of phosphor sur facecoated by Alon is less than 2 percent. In the case of a cool whitephosphor wherein the average phosphor particle size is 10 microns, thethickness required would be 30 microns and the percentage coatingeffected by M 0 would be even less. Clumping of the colloidal aluminafurther reduces the proportion of the phosphor particles coated orcovered. It is apparent from the foregoing data that the quantity ofaluminum oxide in the postcoat according to our invention is only aminor fraction of the quantity required to provide complete coverage ofthe phosphor particles.

The discontinuous, that is porous and pervious nature of the colloidalalumina postcoat applied over the phosphor layer in accordance with theinvention has been confirmed by photo-micrographs taken by scanningelectron microscope.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A fluorescent lamp comprising an elongated vitreous tube containingan ionizable medium including mercury vapor and having electrodes sealedinto its ends, a coating of phosphor on a major portion of the insidesurface of said tube, and a postcoat of finely divided aluminum oxide ofsubmicron size forming a porous discontinuous coating on the particlesof said phosphor layer to improve maintenance and reduce enddiscloration, the quantity of aluminum oxide in said postcoat being buta small fraction of the quantity required to achieve complete coverageof the phosphor particles, the weight of said aluminum oxide postcoatbeing in the range of 10 to I60 micrograms per square centimeter.

2. A lamp as in claim 1 wherein a particulate reflec tor layer underliesthe phosphor layer.

3. A lamp as in claim 1 wherein a particulate reflector lamp underliesthe phosphor layer and an aperture is provided therein extending over aminor portion of the tube periphery.

4. A lamp as in claim 1 wherein a particulate reflector layer of "H0underlies the phosphor layer and an aperture is provided thereinextending over a minor portion of the tube peripheryv 5. A lamp as inclaim 1 of the aperture type compris ing a reflector layer of TiOunderlying a Zn SiO phosphor layer, an aperture therein extending over aminor portion of the tube periphery, and a postcoat of A1 0 of about 40micrograms/cm over the phosphor layer. =k

1. A FLUORESCENT LAMP COMPRISING AN ELONGATED VITREOUS TUBE CONTAININGAN IONIZABLE MEDIUM INCLUDING MERCURY VAPOR AND HAVING ELECTRODES SEALEDINTO ITS ENDS, A COATING OF PHOSPHOR ON A MAJOR PORTION OF THE INSIDESURFACE OF SAID TUBE, AND A POSTOCOAT OF FINELY DIVIDED ALUMINUM OXIDEOF SUBMICRON SIZE FORMING A POROUS DISCONTINUOUS COATING ON THEPARTICLES OF SAID PHOSPHOR LAYER TO IMPROVE MAINTENANCE AND REDUCE ENDDISCLORATION, THE QUANTITY OF ALUMINUM OXIDE IN SAID POSTCOAT BEING BUTA SMALL FRACTION OF THE QUANTITY REQUIRED TO ACHIEVE
 2. A lamp as inclaim 1 wherein a particulate reflector layer underlies the phosphorlayer.
 3. A lamp as in claim 1 wherein a particulate reflector lampunderlies the phosphor layer and an aperture is provided thereinextending over a minor portion of the tube periphery.
 4. A lamp as inclaim 1 wherein a particulate reflector layer of TiO2 underlies thephosphor layer and an aperture is provided therein extending over aminor portion of the tube periphery.
 5. A lamp as in claim 1 of theaperture type comprising a reflector layer of TiO2 underlying a Zn2SiO4phosphor layer, an aperture therein extending over a minor portion ofthe tube periphery, and a postcoat of Al2O3 of about 40 micrograms/cm2over the phosphor layer.